Digital image processing devices, such as digital cameras, use automatic features to increase the quality of an image, such as the preview screen on the digital camera as well as the recorded image and recorded video. This includes the 3A features which refers to automatic white balancing (AWB), automatic focus (AF), and automatic exposure control (AEC). Exposure is the amount of incident light captured by a sensor, and which may be adjusted by adjusting the camera's aperture size and shutter speed as well as ND filter control and flash power, some of which may be electronic systems rather than mechanical devices. ND filter refers to a Neutral Density filter that is sometimes used with mechanical shutters when the mechanical shutter is not fast enough for the brightest illumination conditions. The AEC also may calculate analog gain, and digital gain when present, that amplify the raw image signal that results from the used exposure time. Together the exposure parameters determine a total exposure time referred to herein as the total exposure. The gains impact a signal level, or brightness, of the RAW image that comes out from the camera sensor. If the total exposure is too short, images will appear darker than the actual scene, which is called under-exposure. An image signal can even be so under-exposed as to become lost if the signal is less than a noise floor, or is quantized to zero. On the other hand, if the total exposure is too long, the output images will appear brighter than the actual scene, which is called over-exposure. An image signal may even be lost due to over-exposure when the signal becomes saturated, such as when the signal value exceeds a full well capacity of the pixels, or when the analog or digital gains cause the signal value to exceed a maximum digital value. The signal can even be lost if it becomes saturated. Both cases may result in a loss of detail resulting in a poor quality image.
Automatic exposure control is used to compute the correct exposure necessary to obtain an acceptable display image from the digital camera, such as for the viewfinder or preview screen on the camera, recorded video, and/or still images. Such computations also may be used to adjust the exposure settings for capturing subsequent images with the camera. Thus, the timing of the transmission of parameters between a processor performing the AEC calculations and a camera controller are very important. If delay in transmission occurs, a flicker or jump in brightness in a digital camera screen may be visible to a user via viewfinder screen or recorded video, and an image may be captured or recorded with the wrong exposure. Thus, it is desirable to have an automatic exposure control that maintains high quality images and video.